Template Attribute Language (TAL)¶
The Template Attribute Language (TAL) is an attribute language used to create dynamic XML-like content. It allows elements of a document to be replaced, repeated, or omitted.
An attribute language is a programming language designed to render documents written in XML markup. The input XML must be well-formed. The output from the template is usually XML-like but isn’t required to be well-formed.
The statements of the language are document tags with special attributes, and look like this:
<p namespace:command="argument">Some Text</p>
In the above example, the attribute namespace:command="argument"
is the statement, and the entire paragraph tag is the statement’s
element. The statement’s element is the portion of the document on
which this statement operates.
Each statement has three parts: the namespace prefix, the name, and the argument. The prefix identifies the language, and must be introduced by an XML namespace declaration in XML and XHTML documents, like this:
xmlns:namespace="http://example.com/namespace"
The statements of TAL are XML attributes from the TAL namespace. These attributes can be applied to an XML or HTML document in order to make it act as a template.
The TAL namespace URI is currently defined as:
xmlns:tal="http://xml.zope.org/namespaces/tal"
This is not a URL, but merely a unique identifier. Do not expect a browser to resolve it successfully. This definition is required in every file that uses ZPT. For example:
<div xmlns="http://www.w3.org/1999/xhtml"
xmlns:tal="http://xml.zope.org/namespaces/tal">
.... rest of the template here ...
</div>
All templates that you use ZPT in must include the
xmlns:tal="http://xml.zope.org/namespaces/tal"
attribute on some
top-level tag.
Statements¶
A TAL statement has a name (the attribute name) and an argument
(the attribute value). For example, a tal:content
statement might
look like tal:content="string:Hello"
. The element on which a
statement is defined is its statement element. Most TAL
statements are expressions, but the syntax and semantics of these
expressions are not part of TAL.
Note
TALES is used as the expression language for the “stuff in the quotes” typically. TALES is documented separately.
These are the available TAL statements:
tal:attributes
- dynamically change element attributes.tal:define
- define variables.tal:condition
- test conditions.tal:content
- replace the content of an element.tal:omit-tag
- remove an element, leaving the content of the element.tal:repeat
- repeat an element.tal:replace
- replace the content of an element and remove the element leaving the content.
Order of Operations¶
When there is only one TAL statement per element, the order in which they are executed is simple. Starting with the root element, each element’s statements are executed, then each of its child elements is visited, in order, to do the same.
Any combination of statements may appear on the same element, except
that the tal:content
and tal:replace
statements may not be
used on the same element.
TAL does not use use the order in which statements are written in the tag to determine the order in which they are executed. When an element has multiple statements, they are executed in this order:
tal:define
tal:condition
tal:repeat
tal:content
ortal:replace
tal:omit-tag
tal:attributes
There is a reasoning behind this ordering. Because users often want
to set up variables for use in other statements contained within this
element or subelements, tal:define
is executed first.
tal:condition
follows, then tal:repeat
, then tal:content
or tal:replace
. Finally, before tal:attributes
, we have
tal:omit-tag
(which is implied with tal:replace
).
tal:attributes¶
Replace element attributes
Syntax¶
tal:attributes
syntax:
argument ::= attribute_statement [';' attribute_statement]*
attribute_statement ::= attribute_name expression
attribute_name ::= [namespace-prefix ':'] Name
namespace-prefix ::= Name
Description¶
The tal:attributes
statement replaces the value of an attribute
(or creates an attribute) with a dynamic value. The
value of each expression is converted to a string, if necessary.
Note
You can qualify an attribute name with a namespace prefix,
for example html:table
, if you are generating an XML document
with multiple namespaces.
If an attribute expression evaluates to null
, then that attribute is deleted from the statement element.
If the expression evaluates to the symbol default
(a symbol which is always available when evaluating attributes), its value is defined as the default static attribute value.
If you use tal:attributes
on an element with an active
tal:replace
command, the tal:attributes
statement is ignored.
If you use tal:attributes
on an element with a tal:repeat
statement, the replacement is made on each repetition of the element,
and the replacement expression is evaluated fresh for each repetition.
Examples¶
Replacing a link:
<a href="/sample/link.html"
tal:attributes="href context.url()">
Replacing two attributes:
<textarea rows="80" cols="20"
tal:attributes="rows request.rows();cols request.cols()">
tal:condition¶
Conditionally insert or remove an element
Description¶
Thetal:condition
statement includes the statement element in the template only if the condition is met, and omits it otherwise. If its expression evaluates to a true value, then normal processing of the element continues, otherwise the statement element is immediately removed from the template. For these purposes, the valuenothing
is false, anddefault
has the same effect as returning a true value.
Note
SharpTAL considers null, zero, empty strings,
empty sequences, empty dictionaries false; all other
values are true, including default
.
Examples¶
Test a variable before inserting it:
<p tal:condition="request.message"
tal:content="request.message">
message goes here
</p>
Testing for odd/even in a repeat-loop:
<div tal:repeat="item Enumerable.Range(0, 10)">
<p tal:condition='repeat["item"].even'>Even</p>
<p tal:condition='repeat["item"].odd'>Odd</p>
</div>
tal:content¶
Replace the content of an element
Description¶
Rather than replacing an entire element, you can insert text or
structure in place of its children with the tal:content
statement.
The statement argument is exactly like that of tal:replace
, and is
interpreted in the same fashion. If the expression evaluates to
null
, the statement element is left childless. If the
expression evaluates to default
, then the element’s contents are
unchanged.
The default replacement behavior is text
, which replaces
angle-brackets and ampersands with their HTML entity equivalents. The
structure
keyword passes the replacement text through unchanged,
allowing HTML/XML markup to be inserted. This can break your page if
the text contains unanticipated markup (eg. text submitted via a web
form), which is the reason that it is not the default.
Examples¶
Inserting the user name:
<p tal:content="user.getUserName()">Fred Farkas</p>
Inserting HTML/XML:
<p tal:content="structure context.getStory()">marked <b>up</b>
content goes here.</p>
tal:define¶
Define variables
Syntax¶
tal:define
syntax:
argument ::= attribute_statement [';' attribute_statement]*
attribute_statement ::= [context] variable_name expression
context ::= global | local | nonlocal
variable_name ::= Name
Description¶
The tal:define
statement defines variables.
When you define a local variable in a statement element, you can use that variable in that element and the elements it contains.
If the expression associated with a variable evaluates to null
,
then that variable has the value null
, and may be used as such
in further expressions. Likewise, if the expression evaluates to
default
, then the variable has the value default
, and may be
used as such in further expressions.
Examples¶
Defining a global variable:
<tal:tag tal:define='global company_name '"My Company"'>
Defining a local variable:
<tal:tag tal:define='company_name "My Company"'>
Defining two local variables, where the second depends on the first:
<tal:tag tal:define="mytitle context.title; tlen mytitle.Length">
Declare that the listed identifiers refers to previously bound variables in the nearest enclosing scope:
<p tal:define="mytitle context.title">
<tal:tag tal:define="nonlocal mytitle context.new_title">
</p>
tal:omit-tag¶
Remove an element, leaving its contents
Description¶
The tal:omit-tag
statement leaves the contents of an element in
place while omitting the surrounding start and end tags.
If the expression evaluates to a false value, then normal processing of the element continues and the tags are not omitted. If the expression evaluates to a true value, or no expression is provided, the statement element is replaced with its contents.
Note
null, zero, empty strings,
empty sequences, empty dictionaries are false; all other
values are true, including default
.
Examples¶
Unconditionally omitting a tag:
<div tal:omit-tag="" comment="This tag will be removed">
<i>...but this text will remain.</i>
</div>
Conditionally omitting a tag:
<b tal:omit-tag="bold == false">I may be bold.</b>
The above example will omit the b
tag if the variable bold
is false.
Creating ten paragraph tags, with no enclosing tag:
<span tal:repeat="n Enumerable.Range(0, 10)" tal:omit-tag="">
<p tal:content="n">1</p>
</span>
tal:repeat¶
Repeat an element
Description¶
The tal:repeat
statement replicates a sub-tree of your document
once for each item in a sequence. The expression should evaluate to a
sequence. If the sequence is empty, then the statement element is
deleted, otherwise it is repeated for each value in the sequence. If
the expression is default
, then the element is left unchanged, and
no new variables are defined.
The variable_name
is used to define a local variable and a repeat
variable. For each repetition, the local variable is set to the
current sequence element, and the repeat variable is set to an
iteration object.
Repeat Variables¶
You use repeat variables to access information about the current
repetition (such as the repeat index). The repeat variable has the
same name as the local variable, but is only accessible through the
built-in variable named repeat
.
The following information is available from the repeat variable:
index
- repetition number, starting from zero.number
- repetition number, starting from one.even
- true for even-indexed repetitions (0, 2, 4, ...).odd
- true for odd-indexed repetitions (1, 3, 5, ...).start
- true for the starting repetition (index 0).end
- true for the ending, or final, repetition.length
- length of the sequence, which will be the total number of repetitions.letter
- repetition number as a lower-case letter: “a” - “z”, “aa” - “az”, “ba” - “bz”, ..., “za” - “zz”, “aaa” - “aaz”, and so forth.Letter
- upper-case version of letter.roman
- repetition number as a lower-case roman numeral: “i”, “ii”, “iii”, “iv”, “v”, etc.Roman
- upper-case version of roman.
You can access the contents of the repeat variable using dictionary, e.g. repeat["item"].start
.
Examples¶
Iterating over a sequence of strings:
<p tal:repeat='txt new List<string>() { "one", "two", "three" }'>
<span tal:replace="txt" />
</p>
Inserting a sequence of table rows, and using the repeat variable to number the rows:
<table>
<tr tal:repeat="item here.cart">
<td tal:content='repeat["item"].number'>1</td>
<td tal:content="item.description">Widget</td>
<td tal:content="item.price">$1.50</td>
</tr>
</table>
Nested repeats:
<table border="1">
<tr tal:repeat="row Enumerable.Range(0, 10)">
<td tal:repeat="column Enumerable.Range(0, 10)">
<span tal:define='x repeat["row"].number;
y repeat["column"].number;
z x * y'
tal:replace="string:${x} * ${y} = ${z}">1 * 1 = 1</span>
</td>
</tr>
</table>
Insert objects. Separates groups of objects by type by drawing a rule between them:
<div tal:repeat="object objects">
<h2 tal:condition='repeat["object"].first.meta_type'
tal:content="object.type">Meta Type</h2>
<p tal:content="object.id">Object ID</p>
</div>
Note
the objects in the above example should already be sorted by type.
tal:replace¶
Replace an element
Description¶
The tal:replace
statement replaces an element with dynamic
content. It replaces the statement element with either text or a
structure (unescaped markup). The body of the statement is an
expression with an optional type prefix. The value of the expression
is converted into an escaped string unless you provide the ‘structure’ prefix. Escaping consists of converting &
to
&amp;
, <
to &lt;
, and >
to &gt;
.
If the expression evaluates to null
, the element is simply removed. If the value is default
, then the element is left unchanged.
Examples¶
Inserting a title:
<span tal:replace="context.title">Title</span>
Inserting HTML/XML:
<div tal:replace="structure table" />